Several methods are known for commercial production of acetic acid, which comprise, for example, (a) oxidizing acetaldehyde, (b) directly oxidizing petroleum naphtha, butane, propane or the like, (c) purifying acetic acid obtained as a byproduct in the synthesis of epoxides from olefins and peracetic acid or from unsaturated alcohols and peracetic acid, or (d) contacting methanol with carbon monoxide for carbonylation in the presence of a transition metal catalyst such as rhodium and a halogen-containing organic promoter such as methyl iodide.
Methods known for commercial production of acetic anhydride comprise, for example, (e) reacting acetic acid with ketene prepared by pyrolysis of acetic acid, or (f) contacting methyl acetate or dimethyl ether with carbon monoxide for carbonylation in the presence of a transition metal catalyst such as rhodium and a halogen-containing organic promoter such as methyl iodide.
In particular, the method of producing acetic acid or acetic anhydride by carbonylation with carbon monoxide is a very advantageous commercial production method. As another convenient method, there may be mentioned the method of producing acetic acid and acetic anhydride simultaneously which comprises reacting methyl acetate and methanol, or dimethyl ether and methanol, with carbon monoxide.
Generally, however, the product acetic acid or acetic anhydride obtained by such methods as mentioned above contains such impurities as reducing substances, carbonylation products, and halogen compounds. These impurities exert a very serious influence on the result of the potassium permanganate test (commonly known as chameleon test), which is an important index of the quality of acetic acid and acetic anhydride. It is therefore necessary to remove such impurities from the product acetic acid and product acetic anhydride to a great and satisfactory extent.
As other means for removing said impurities than the distillation method which requires a high degree of energy consumption, there are known treatment methods which use ozone, peracetic acid or hydrogen.
Thus, the method disclosed in Japanese Patent laid open No. (JP-A) 02-231448, which uses hydrogen, comprises treating impurity-containing acetic acid with hydrogen at a temperature of about 17.degree. to 200.degree. C. and a hydrogen pressure of about 1 to 30 atmospheres in the presence of 0.01 to 10% by weight of a hydrogenation catalyst and then recovering acetic acid.
The method disclosed in Japanese Patent Publication No.(JP-B) 48-30615, which uses peracetic acid, comprises adding peracetic acid to acetic acid showing a short residence time in the potassium permanganate test and subjecting the resulting mixture to distillation for purification, to give acetic acid of improved quality. Japanese Patent laid open No.(JP-A) 61-56151 discloses a method of preparing acetic acid of improved quality which comprises adding peracetic acid to acetic acid showing a short residence time in the potassium permanganate test, treating the mixture by heating at a temperature within the range of 50.degree. to 120.degree. C. and subjecting the same to distillation for purification.
As for the methods which use ozone, Japanese Patent laid open No.(JP-A) 60-222439 discloses a method comprising treating crude acetic anhydride with an ozone-containing gas. Japanese Patent Publication No.(JP-B) 61-2052 discloses a method of producing acetic acid improved in residence time in the potassium permanganate test which comprises treating acetic acid with a purity of not less than 99% with an ozone-containing gas in the absence of any catalyst metal compound. Further, Japanese Patent laid open No.(JP-A) 01-211548 corresponding to U.S. Ser. No. 137,844, now abandoned, discloses a method comprising contacting acetic acid containing halides, unsaturated compounds and carbonyl compounds as impurities with a sufficient amount of ozone to oxidize said impurities in the presence or absence of a catalyst and recovering purified acetic acid.
Among these treatment methods, those which use ozone or an ozone-containing gas are estimated to be most advantageous from the viewpoints of economy and safety, among others, and are in practical use for the production of acetic acid and acetic anhydride. They are particularly effective in treating impurities close in boiling point to acetic acid or acetic anhydride, hence difficult to remove to a satisfactory extent by distillation.
When, however, unsaturated compounds as impurities are present in an amount of the order several hundred to several thousand parts per million (ppm), no substantial improvement in residence time in the potassium permanganate test will be produced in some cases because of influences of products formed from the impurities upon ozone treatment, although ozone treatment alone can give acetic acid or acetic anhydride of good quality with a long residence time in the potassium permanganate test when unsaturated compounds are present only in trace amounts.